The present invention relates to the joining of individual ceramic components, or pieces, to form a composite ceramic article. Although the present invention will be specifically discussed in regard to articles assembled from components of sintered, or unsintered, silicon carbide, it will be understood that other carbide materials, such as titanium or tungsten carbide or mixtures, may be utilized as the ceramic components.
Silicon carbide has long been known for its hardness, strength, and excellent resistance to oxidation and corrosion. Silicon carbide has a low coefficient of expansion, good heat transfer properties, and maintains high strength at elevated temperatures. In recent years, the art of producing high-density silicon carbide materials by sintering silicon carbide powders under substantially pressureless conditions has been developed. High-density silicon carbide materials find utility in the fabrication of components for turbines, heat exchange units, pumps and other equipment or tools that are exposed to severe corrosion or wear, especially in operations carried out at high temperatures.
Ceramic articles of components thereof may be formed or shaped by various casting or molding processes. Suitable forming or shaping processes which are well known in the art, for example, cold pressing, isostatic forming, slip casting, extrusion, injection or transfer molding or tape casting, may be utilized to initially form the components of the present invention.
In many forming or shaping operations, it is desirable, or more economical, to form one component by one molding method and another or other components by different molding methods and subsequently joining the components to form a composite article. In some cases, it is not feasible to case or mold the entire article as a unit. In such cases, the components are separately formed and subsequently joined to produce composite articles of complex shapes or compositions.
The present invention provides a method of joining formed components which may be entirely of unsintered metal carbides, or green bodies, entirely of sintered metal carbides, or mixtures of sintered and unsintered metal carbides.
Silicon carbide bodies having high density and high strength are produced by sintering particulate silicon carbide in the form of the article. More recently, the art of pressureless sintering of silicon carbide has been applied to full-scale commercial processes. In such processes, a green body of unsintered silicon carbide is formed by molding or shaping a mixture of particulate silicon carbide, excess carbon and a sintering aid. The formed green body is sintered under substantially pressureless conditions at temperatures between about 2050.degree. and about 2100.degree. C. for a period of from about 20 minutes to about 30 minutes to produce a sintered silicon carbide article. The sintering process produces a product having the molded shape, but with slightly smaller dimensions, because of shrinking during the sintering process. Various compounds of boron or beryllium have been found useful as sintering or densification aids. Such aids are usually added to the ceramic material powder in amounts ranging between about 0.3 and about 5.0 percent by weight of boron or beryllium, based on the weight of the mixture. The sintering aid may be in the form of elemental boron or beryllium or in the form of boron- or beryllium-containing compounds. Boron is the preferred additive for reasons of handling and performance. Boron is commonly utilized in the form of boron carbide. Examples of boron-containing silicon carbide powders and methods of producing sintered articles are described in U.S. Pat. application Ser. No. 584,226, filed June 5, 1975, now U.S. Pat. No. 4,312,954 and U.S. Pat. Nos. 4,179,299 and 4,124,667, the teachings of which are hereby incorporated herein by reference. With respect to beryllium, see U.S. Pat. No. 4,172,109.
The present invention provides a method of utilizing sintered or unsintered silicon carbide bodies as components of articles having complex shapes, or as components of composite articles having surfaces or parts which vary in chemical or physical properties.